Max speed of acme leadscrew
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  1. #1
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    Default Max speed of acme leadscrew

    I'm whipping up a general purpose shop press to do the operations our press brakes aren't currently well suited for. This is includes pressing in bearings, dishing plate, bending thick steel, stamping logos, and anything else exceeding 25 tons per foot.

    I'm making it out of parts I have.

    I have a pair of 5" hydraulic cylinders and a rotary flow divider that should allow the cylinders to pull 50 tons combined. These will pull on two 1-1/2"-4 acme threaded rods, with the ram attached to the nuts. The screws will be spun to rapid travel the ram up and down, and then the cylinders will pull the entire assembly down the rest of the way. The only thing holding ram alignment is these screws in the bushings they rotate and slide in.


    The reason to build it like this is that it's easy to make and allows for good positioning accuracy with simple components, and will be less heavy than it would otherwise be.

    Is 800 RPM too much for these screws? It will be periodically lubricated but will have two steel nuts per screw to handle the load.

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    Just for reference, I recently finished building a machine that utilized a 3/4"x 6 hi-lead acme screw that turns 700 RPM, and that combination worked. Your screw diameter is huge and you're using two nuts so I would guess you would be fine. Anything over 700 RPM on the unit I built and things start to get scary, but the screw is almost 7 feet long.

    Stuart

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    You're dealing with a critical rotating shaft speed issue, and there's calculators for that (bless the Internet):

    critical speed calculator

    Using some simple values (1.13" for minor diameter, 30" free length, safety factor 2, "Fixed/Free" shaft support), I got a critical speed of ~750, which indicates your 800 is probably fine.

    This is not engineering advice, if the shaft decides to bend and kill everyone in a 20ft radius it's not my fault...

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    Quote Originally Posted by Milland View Post
    You're dealing with a critical rotating shaft speed issue, and there's calculators for that (bless the Internet):

    critical speed calculator

    Using some simple values (1.13" for minor diameter, 30" free length, safety factor 2, "Fixed/Free" shaft support), I got a critical speed of ~750, which indicates your 800 is probably fine.

    This is not engineering advice, if the shaft decides to bend and kill everyone in a 20ft radius it's not my fault...
    Honestly I wasn't worried about critical speed so much as the speed of the steel nut riding on the steel thread. That critical speed is surprisingly low though.

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    Quote Originally Posted by Strostkovy View Post
    Honestly I wasn't worried about critical speed so much as the speed of the steel nut riding on the steel thread. That critical speed is surprisingly low though.
    I can't vouch for the accuracy of the calculator, I use them for reference, not gospel. But I did throw a "2" safety factor in, so presumably it's closer to 1500 actual critical speed.

    On a steel nut/steel rod aspect, keep it lubed, examine every so often for wear particles. No debris, no problem. Shiny tiny bits, new choice of grease or change the nuts to bronze.

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    Maybe consider arranging a felt washer to run in the thread and keeping it oil soaked to continuously lubricate the threads. Way back I had a large acme nut one through my hands with one end of the thread cut away to hold a felt oiling ring. But that was a low speed device, maybe out of a car lift. Keeping an oiling ring in place at 800 rpm sounds challenging.

    Clive
    Last edited by Clive603; 02-17-2021 at 01:55 PM.

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    Interesting, I figured you simply use the threaded rod as the "bottom stop",
    and let the hydraulics do all the work.

    I looked at a press brake (25 ton) and it has the cylinders pulling the top down, and a stop screw on each side, right next to the cylinders.
    IIRC they were normal thread, and maybe 1 1/2" dia.

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    I retro'd my old 65 ton Verson press brake with a Servo style Bridgeport power feed to run the shut height nuts.

    Works great, lots of torque in a small package, variable speed and the shaft passes through so you can put a handwheel or a BP handle on the shaft for fine adjustment. The cheap chine ones (what I used) are like $100 on ebay.

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    This thread was a learning experience for me. I built a machine utilizing a acme screw and a variable speed drive. I drove the screw until it started to sing a song, backed it of a hair and set that as the max speed..didn't know about the site that Milland linked. After visiting said site, I input my data, and Bob really was my uncle, the calculated max speed on the linked site and the max speed I derived from hands-on matched to a tee.

    Looks like there are several ways to skin the feline..the scientific method or the cowboy way. Being technically challenged I usually go with the cowboy way.

    Stuart

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